Availability template for template helper

Please add “Availability template” to a template helper. Currently, template helpers have to have null checks for every sensor they use, making the templates hardly readable. This could be avoided if we have a way to set up availability template (which I believe exists in data model, but is missing from GUI). That way, all the checks would be on one place - availability template - and the template to get the actual sensor value would have only the logic to get the value, which looks much cleaner.

Duplicate of